1. Field of the Invention
The present invention relates to a complex objective lens in which an objective lens is combined with a hologram that is a diffraction element; an optical head for condensing light beams having a plurality of wavelengths onto an optical disk via the complex objective lens to record, reproduce, or delete information; an optical information apparatus in which the optical head is mounted; and a computer, an optical disk player, a car navigation system, an optical disk recorder and an optical disk server to which the optical information apparatus is applied.
2. Description of the Related Art
An optical memory technique using an optical disk having a pit-shaped pattern as a storage medium with a high density and a large capacity is being put to practical use while extending the range of uses to a digital audio disk, a video disk, a document file disk, and a data file. A function of recording/reproducing information with respect to an optical disk satisfactorily with high reliability, using a minutely condensed light beam, is roughly classified into a condensing function of forming a minute spot of a diffraction limit, focal point control (focus servo) and tracking control of an optical system, and detection of a pit signal (information signal).
Recently, because of the advancement of an optical system design technique and a decrease in wavelength of a semiconductor laser that is a light source, a high-density optical disk having a storage capacity larger than that of the prior art is being developed. As an approach to larger densities, an increase in a numerical aperture (NA) on an optical disk side of a condensing optical system that condenses a light beam minutely onto an optical disk is being studied. In this case, there is a problem that the amount of aberration caused by the inclination (i.e., tilt) of an optical axis is increased. When a NA is increased, the amount of aberration occurring with respect to a tilt also is increased. In order to prevent this, the thickness (base thickness) of a substrate of an optical disk should be made thinner.
A compact disk (CD) that may be a first generation optical disk uses infrared light (wavelength λ3: 780 nm to 820 nm) and an objective lens with a NA of 0.45, and has a base thickness of 1.2 mm. A DVD that is a second generation optical disk uses red light (wavelength λ2: 630 nm to 680 nm; standard wavelength: 660 nm) and an objective lens with a NA of 0.6, and has a base thickness of 0.6 mm. Furthermore, a third generation optical disk (hereinafter, referred to as a BD (Blue-ray Disk) uses blue light (wavelength λ1: 390 nm to 415 nm; standard wavelength: 405 nm) and an objective lens with a NA of 0.85, and has a base thickness of 0.1 mm. In the present specification, the base thickness refers to a thickness from a surface of an optical disk (or an information medium) upon which a light beam is incident to an information recording surface.
Thus, the base thickness of an optical disk is decreased with an increase in density. In terms of economical points and a space occupied by an apparatus, there is a demand for an optical information apparatus capable of recording/reproducing information with respect to optical disks having different base thicknesses and recording densities. In order to achieve this, an optical head is required that is provided with a condensing optical system capable of condensing a light beam to a diffraction limit onto optical disks having different base thicknesses.
Furthermore, in the case where information is recorded/reproduced with respect to an optical disk having a thick base material, it is necessary to condense a light beam onto a recording surface that is positioned on a deeper side of the disk surface. Therefore, a focal length needs to be made larger.
JP 7(1995)-98431 A discloses a configuration intended to realize an optical head that records/reproduces information with respect to optical disks having different base thicknesses. This configuration will be described as a first conventional example with reference to FIGS. 25A and 25B.
In FIGS. 25A and 25B, reference numerals 40 and 41 denote an objective lens and a hologram, respectively. The hologram 41 is provided with a concentric grating pattern on a substrate transparent to an incident light beam 44.
The objective lens 40 has an numerical aperture NA of 0.6 or more, and as shown in FIG. 25A, is designed so as to allow 0th-order diffracted light 42 that passes through the hologram 41 without being diffracted to form a condensed spot of a diffraction limit on an optical disk 10, for example, having a base thickness (t2) of 0.6 mm. Furthermore, FIG. 25B shows that a condensed light spot of a diffraction limit can be formed on an optical disk 11 having a larger base thickness (t1) (i.e., 1.2 mm). In FIG. 25B, +1st-order diffracted light 43 diffracted by the hologram 41 is condensed onto the optical disk 11 by the objective lens 40. Herein, the +1st-order diffracted light 43 is subjected to aberration correction so as to be condensed to a diffraction limit through a substrate with a thickness t1.
Thus, by combining the hologram 41 that diffracts incident light with the objective lens 40, a 2-focal point lens is realized, which is capable of forming a condensed light spot that is condensed to a diffraction limit on the optical disks 10, 11 having different base thicknesses (t1 and t2), using diffracted light of different orders. Furthermore, it also is disclosed that, conversely to the above, the hologram 41 is designed so as to have a convex lens action, and 0th-order diffracted light is used for the optical disk 11 with the base thickness t1, and +1st-order diffracted light is used with respect to the optical disk 10 with the base thickness t2, whereby a fluctuation in a focal point position can be reduced with respect to a wavelength fluctuation during recording/reproducing of information with respect to the optical disk having the base thickness t2.
There also is a disclosure of a configuration intended for compatible reproducing of information with respect to optical disks having different kinds, using light beams having a plurality of wavelengths. As a second conventional example, a configuration in which a wavelength selection phase plate is combined with an objective lens is disclosed by JP 10(1998)-334504 A and Session We-C-05 of ISOM2001 (page 30 of the preprints). The configuration disclosed by Session We-C-05 of ISOM2001 (Page 30 of the preprints) will be described with reference to FIGS. 26, 27A and 27B.
FIG. 26 is a cross-sectional view showing a schematic configuration of an optical head as the second conventional example. In FIG. 26, parallel light output from a blue light optical system 51 having a blue light source (not shown) with a wavelength λ1 of 405 nm passes through a beam splitter 161 and a wavelength selection phase plate 205 and is condensed onto an information recording surface of an optical disk 9 (third generation optical disk: BD) with a base thickness of 0.1 mm by an objective lens 50. The light reflected from the optical disk 9 follows a reverse path and is detected by a detector (not shown) of the blue light optical system 51. On the other hand, divergent light output from a red light optical system 52 having a red light source (not shown) with a wavelength λ2 of 660 nm is reflected by the beam splitter 161, passes through the wavelength selection phase plate 205, and is condensed onto the information recording surface of an optical disk 10 (second generation optical disk: DVD) with a base thickness of 0.6 mm by the objective lens 50. Light reflected from the optical disk 10 follows a reverse path and is detected by a detector (not shown) of the red light optical system 52.
The objective lens 50 is designed so as to allow parallel light to pass through the optical disk 9 with the base thickness of 0.1 mm to be condensed. Therefore, for recording/reproducing of information with respect to the DVD with the base thickness of 0.6 mm, spherical aberration is caused by the difference in base thickness. In order to correct the spherical aberration, a light beam output from the red light optical system 52 is formed into dispersed light, and the wavelength selection phase plate 205 is used. When dispersed light is incident upon the objective lens 50, new spherical aberration occurs. Therefore, the spherical aberration caused by the difference in base thickness is cancelled by the new spherical aberration, and a wavefront is corrected by the wavelength selection phase plate 205.
FIGS. 27A and 27B respectively are a plan view and a cross-sectional view of the wavelength selection phase plate 205 in FIG. 26. The wavelength selection phase plate 205 is configured with a level difference 205a between heights h and 3h, assuming that the refractive index at a wavelength λ1 is n1, and h=λ1/(n1−1). The optical path difference caused by the level difference of the height h is a use wavelength λ1, which corresponds to a phase difference 2π. This case is the same as the phase difference of 0. Therefore, the level difference of the height h does not influence the phase distribution of a light beam with a wavelength λ1, and does not influence recording and reproducing of information with respect to the optical disk 9 (FIG. 26). On the other hand, assuming that the refractive index of the wavelength selection phase plate 205 at a wavelength λ2 is n2, h×(n2−1)/λ2≈0.6, i.e., an optical path difference that is not an integral multiple of a wavelength occurs. The above-mentioned aberration correction is performed by using a phase difference caused by the optical path difference.
Furthermore, as a third conventional example, JP 11(1999)-296890 A and the like disclose a configuration in which a plurality of objective lenses are switched mechanically.
Furthermore, as a fourth conventional example, JP 11(1999)-339307 A discloses a configuration in which a mirror having a reflective surface with different radii of curvature also functions as a rising mirror (changing the direction of light from a horizontal direction to a vertical direction so that the light is incident upon an optical disk) that bends an optical axis.
As a fifth conventional example, JP 2000-81566 A discloses a configuration in which a refraction type objective lens is combined with a hologram in the same way as in the first conventional example, and the difference in base thickness is corrected by using chromatic aberration caused by diffracted light of the same order as that of light having a different wavelength
As a sixth conventional example, as shown in FIG. 28, a configuration in which a refraction type objective lens 281 is combined with a hologram 282 having a diffraction surface and a refraction surface is described in “BD/DVD/CD Compatible Optical Pickup Technique” by Sumito Nishioka (Extended Abstracts (50th Spring Meeting); The Japan Society of Applied Physics, 27p-ZW-10 (Kanagawa University, March 2003)) (published after filing of the priority application of the present application). In the sixth conventional example, the hologram 282 is allowed to generate +2nd-order diffracted light with respect to a blue light beam and +1st-order diffracted light with respect to a red light beam, whereby chromatic aberration correction is performed. Furthermore, dispersed light is allowed to be incident upon the hologram 282 and the objective lens 281 with respect to a blue light beam, and converged light is allowed to be incident upon them with respect to a red light beam, whereby spherical aberration caused by a difference in base thickness is corrected.
The above-mentioned first conventional example proposes at least the following three technical ideas. First, the compatibility of optical disks having different base thicknesses is realized by using the diffraction of a hologram. Second, the design of an inner/outer periphery is changed to form condensed light spots having different NAs. Third, a focal point position of a condensed light spot is changed with respect to optical disks having different base thicknesses by using the diffraction of a hologram. These technical ideas do not limit the wavelength of light to be emitted by a light source.
Herein, a DVD that is the second generation optical disk includes a two-layer disk having two recording surfaces. The recording surface (first recording surface) on a side closer to an objective lens needs to allow light to pass through to a surface away from the objective lens, so that its reflectivity is set at about 30%. However, this reflectivity is guaranteed only with respect to red light, and is not guaranteed at the other wavelengths. Therefore, in order to exactly reproduce information from a DVD, it is required to use red (wavelength λ2=630 nm to 680 nm) light. Furthermore, in recording/reproducing of information with respect to a BD that is the third-generation optical disk, it is required to use blue (wavelength λ1=390 nm to 415 nm) light so as to decrease the diameter of a condensed light spot sufficiently. Thus, the first conventional example does not disclose the configuration in which a light use efficiency is enhanced further when different kinds of optical disks are made compatible using, in particular, red light and blue light.
Furthermore, the first conventional example discloses an example in which a hologram is formed in a convex lens type, and +1st-order diffracted light is used, whereby the movement of a focal point position due to a change in wavelength is reduced with respect to one kind of optical disk. However, the first conventional example does not disclose a scheme of reducing simultaneously the movement of a focal point position caused by a change in wavelength with respect to each of at least two kinds of optical disks.
The second conventional example uses a wavelength selection phase plate as a compatible element. When information is recorded/reproduced with respect to a disk having a large base thickness, a recording surface is positioned away from an objective lens by the base thickness. Therefore, it is required to increase a focal length. The focal length can be increased by providing the compatible element with a lens power. However, the wavelength selection phase plate does not have a lens power. Furthermore, as in the second conventional example, when it is attempted to realize all the lens powers with respect to dispersed red light, a large aberration occurs while an objective lens is moved (e.g., follows a track), resulting in degradation of recording/reproducing characteristics.
In the third conventional example, since objective lenses are switched, it is required to use a plurality of objective lenses, which increases the number of parts, and makes it difficult to miniaturize an optical head. Furthermore, the requirement of a switching mechanism also makes it difficult to miniaturize an apparatus.
In the fourth conventional example, an objective lens is driven independently of a mirror (see FIGS. 4 to 6 in JP 11(1999)-339307 A). However, a light beam is converted from parallel light by a mirror having the above-mentioned radius of curvature. Therefore, when the objective lens is moved by track control or the like, the relative position of the objective lens with respect to an incident light wavefront is changed, aberration occurs, and condensing characteristics are degraded. Furthermore, the reflective surface of a mirror is composed of a surface having a radius of curvature (i.e., a spherical surface); however, the spherical surface is not sufficient for correcting the difference in base thickness and the difference in wavelength, and high-order aberration (5th-order or higher) cannot be reduced sufficiently.
When the fifth conventional example is applied directly to a red light beam and a blue light beam, diffraction efficiencies of the same order cannot be enhanced simultaneously because of a large difference in wavelength, and consequently, the use efficiency of light is decreased.
In the sixth conventional example, dispersed light is allowed to be incident upon a hologram and an objective lens with respect to a blue light beam and converged light to be incident upon them with respect to a red light beam. Therefore, under the condition of focusing i.e., under the condition that a condensed light spot of a diffraction limit is formed on an information recording surface of an optical disk), a light beam reflected to be returned from an optical disk also becomes different in a parallel degree between a blue light beam and a red light beam, and a photodetector for detecting a servo signal cannot be shared between a blue light beam and a red light beam. More specifically, at least two photodetectors are required, which increase the number of parts, leading to an increase in cost.